1. Field of the Invention
The present invention relates to a serial printer with a print head which performs the raster scanning on a print medium, such as a sheet of paper and a printing method utilizing the raster scanning.
2. Description of the Prior Art
A print head of a serial printer has a dot forming element array with a number of dot forming elements arranged in a secondary scanning direction, that is, in the direction of movement of a print medium, such as a paper sheet. With this element array, a number of lines can be simultaneously printed on the paper sheet by means of one main scanning pass of the print head across the paper sheet. In the serial printer using such a print head, particularly an ink jet printer, unevenness in characteristic of individual ink jet nozzles or unevenness in pitch between the adjacent ink jet nozzles raises a problem in view of achieving the high-quality print image.
For solving this problem, U.S. Pat. No. 4,198,642 has proposed a printing method called "interlaced printing" which can provide high-quality printing by making unobtrusive the unevenness in characteristic or pitch of the ink jet nozzles on a printed image. This interlaced printing employs a nozzle array with N ink jet nozzles arranged in the secondary scanning direction at a pitch corresponding to a k-dot pitch in the print resolution, wherein k is an integer no greater than n which represents the number of the ink jet nozzles to be driven among the N ink jet nozzles, and has no prime factors greater than one in common with n. Every time the nozzle array has achieved one main scanning pass across the paper sheet, the secondary scanning, that is, paper feeding, is performed by a distance corresponding to an n-dot pitch.
An example is given hereinbelow.
In this example, a nozzle array with 20 nozzles (N=n=20) arranged at a pitch corresponding to a 3-dot pitch (k=3) is used for printing an image with a resolution of 360 dpi. Expressed in terms of inch, a one-dot pitch is equal to 1/360 inches so that the nozzle pitch, which is the 3-dot pitch, is equal to 3/360 inches, and a paper feeding distance achieved by one-time secondary scanning, that is, a secondary scanning distance, is equal to 20/360 inches.
Accordingly, if the secondary scanning (paper feeding) is performed once, each nozzle is moved by 20/360 inches so that each nozzle is moved to a position one-dot pitch before a position where a seven-ahead nozzle was located upon the last main scanning, that is, one-dot pitch before 21/360 inches ahead. If the secondary scanning is performed once again, each nozzle is moved to a position two-dot pitch before a position where a fourteen-ahead nozzle was located upon the before-last main scanning. In other words, when a line is printed by means of each of the nozzles during a certain main scanning pass, an upper line adjacent to each printed line is printed, during a subsequent main scanning pass, by means of a nozzle which is seven-nozzles apart from the corresponding nozzle. Similarly, a further upper line adjacent to each of the foregoing upper lines is printed, during a further subsequent main scanning pass, by means of a nozzle which is fourteen-nozzles apart from the foregoing corresponding nozzle.
In this fashion, in the interlaced printing, the adjacent lines are always printed by the different nozzles. Thus, even if unevenness exists in nozzle characteristic or nozzle pitch, as described before, to some degree, the unevenness is rendered unobtrusive on the printed image so that the high-quality print image can be achieved.
On the other hand, the foregoing conventional interlaced printing still includes the following problem:
Specifically, in the conventional interlaced printing, incomplete print regions where lines can not be printed fully densely are caused at leading and trailing ends, in the secondary scanning direction, of the paper sheet. In the foregoing example, a region having a width of 38/360 inches extending downward from a position of the uppermost nozzle of the nozzle array during the first main scanning pass becomes an incomplete print region. Similarly, an incomplete print region having an approximately equal width exists at the trailing end of the paper sheet. These incomplete print regions at the leading and trailing ends of the paper sheet can not be used as printing regions. Thus, in the actual interlaced printing, for causing the leading and trailing incomplete print regions to be outside a printing region of the paper sheet where the printing is to be performed, the print head is set to be positioned so as to offset from upper and lower ends of the printing region of the paper sheet by distances corresponding to widths of the incomplete print regions. However, if the offset magnitudes of the print head relative to the paper sheet are so large, gaps between the print head and the paper sheet become unstable so that the quality of the print image is largely lowered.
Particularly, in recent years, such printers are widely available on the market, wherein a paper regulating section for regulating a position of a paper sheet upon printing is offset from a position confronting a print head toward a paper outlet side. In the printer thus structured, a distance from the leading end of the paper sheet to the upper end of the printing region and a distance from the lower end of the printing region to the trailing end of the paper sheet become greater as compared with the case where the paper regulating section is arranged at the position confronting the print head. Thus, the regions are enlarged where the foregoing gaps are unstable. On the other hand, although it has been attempted to increase the number of the nozzles arranged in the nozzle array for enhancing the printing efficiency, this increases the foregoing offset magnitudes of the print head relative to the paper sheet and narrows the high-quality printing region.
As one method for diminishing the offset magnitudes of the print head, it may be considered to employ the normal printing method, instead of the interlaced printing method, in the foregoing incomplete print regions so as to print the lines densely. As appreciated, in the normal printing method, the secondary scanning is performed by the one-dot pitch every time the main scanning is finished, However, according to this method, the print image quality of the region where the normal printing method is applied is largely deteriorated as compared with the print image quality of the region where the interlaced printing method is applied, and thus the boundary therebetween becomes remarkable. This is because, since the adjacent lines corresponding in number to the foregoing integer k are printed by the same nozzles in the normal printing method, the unevenness in nozzle characteristic or nozzle pitch appears clearly on the printed image.